1. Field of the Invention
The present invention is related to nuclear fuel elements and in particular to gas cooled nuclear fuel elements.
2. General Background
Gas-cooled particle bed reactors (PBR's) are cooled by a gas flowing radially inward through the annular particle fuel bed. These reactors use fuel elements formed from particle fuel wherein the fuel particle bed is confined in the annular space between an outer porous cylinder and an inner porous cylinder. The coolant gas flows out of an axial channel defined by the inner porous cylinder. A major concern regarding these types of fuel elements is related to the potential for a failure of the inner porous cylinder (hot frit) to release fuel particles to the reactor exit gas stream. In a closed Brayton cycle, a failure would allow the fuel particles to impinge on the turbine and compressor blades with resulting damage. In an open cycle system, a failure also provides a path for release of the particles to the surrounding environment. A variety of fuel elements and fuel particles are known in the art.
U.S. Pat. No. 3,992,258 discloses coated nuclear fuel particles of low density to accommodate fission gases generated during the use of the fuel particles.
U.S. Pat. No. 3,928,132 discloses a compacted fuel element of annular shape enclosed in a graphite casing constituted by an inner and outer tube. The outer tube is larger than the inner tube and also has a greater coefficient of shrinkage.
U.S. Pat. No. 3,361,638 discloses a nuclear fuel particle having a central core of fissile or fertile material surrounded by a fission-product retentive layer of true pyrolytic graphite.
U.S. Pat. No. 3,311,540 discloses a direct cycle integral vapor generating and superheating reactor having, within each pressure tube, a plurality of concentric annular fuel elements clad in metal such as stainless steel. The coolant passes alternately downwardly and upwardly among the fuel elements from the outer flow passage to the inner flow passage.
U.S. Pat. No. 3,222,773 discloses a process of assembly for arranging cladded tubular and cylindrical nuclear fuel members within each other.
U.S. Pat. No. 3,345,733 discloses a method of constructing a nuclear fuel element of a plurality of part annular plates supported at their longitudinal edges by radial support members to define a series of spaced coaxial tubes.
U.S. Pat. No. 4,759,911 discloses the use of a plurality of progressively sized rigid porous cylinders nested together in coaxial alignment with varying quantities of nuclear fuel distributed on each cylinder.
U.S. Pat. No. 3,074,873 discloses a solid nuclear fuel element comprising a plurality of concentric circular cylinders.
U.S. Pat. No. 3,291,696 discloses a fuel element having a fuel layer sandwiched between a gas-impermeable core member and covering layer, and a cylindrical filler member coaxial with the core member and spaced from the covering layer to define a gap therebetween for the circulation of a purge gas to remove gaseous fission products. Gaseous coolant contacts only the external surface of the core member.
U.S. Pat. No. 3,926,720 discloses a block of moderating material having an array of parallel holes with fissile plutonium in each hole and coolant channels for heat removal.
U.S. Pat. No. 4,022,663 discloses pressed spherical fuel elements made of graphite for high temperature reactors.
U.S. Pat. No. 4,678,629 discloses an annular fuel pellet formed from an annular outer pellet and a cylindrical inner pellet.
U.S. Pat. Nos. 2,985,576; 3,138,534; 3,422,523; and 3,753,854 disclose a variety of fuel elements and are representative of the known art.
As the known art does not address the aforementioned problem, there exists a need for a gas cooled nuclear fuel element which significantly reduces the possibility of the release of fuel particles from the fuel element.